1. Field of the Invention
This invention relates to curable organosiloxane compositions. More particularly, this invention relates to organosiloxane compositions containing as the main ingredient a high consistency, gum polyorganosiloxane. The present compositions are particularly suitable for processing by extrusion molding and by pressure-molding techniques such as press molding, transfer molding, and injection molding. The compositions are characterized by an absence of mold staining in combination with a resistance to scorching, good mold release and long term storage stability.
2. Background Information
Organosiloxane compositions characterized by those skilled in the art as millable typically contain as the major organosiloxane ingredient at least one gum organopolysiloxane. The molding methods generally chosen for these millable organosiloxane compositions are extrusion molding and pressure-molding methods such as press molding, transfer molding, and injection molding. The curing agents typically used in these moldable compositions are organic peroxides. The advantages associated with the use of these curing agents are excellent storage stability in the vicinity of room temperature and a relatively long working time at elevated temperatures, which facilitates fabrication of these compositions using conventional molding techniques.
Curable compositions containing organic peroxides have some disadvantages when used in pressure molding applications. These disadvantages include poor mold release properties, resulting in cracking or tearing of the rubber during demolding, and mold staining due to incomplete curing in the regions adjacent to the molding's parting line. Mold staining is believed to result from an inhibition of curing by atmospheric oxygen.
Disadvantages associated with extrusion molding of high-consistency peroxide curable organosiloxane compositions include development of surface tackiness which, like mold staining, is due to an inhibition of curing at the molding's surface by atmospheric oxygen. A dusting agent such as talc is typically used to prevent surface tackiness.
The methods that have been proposed to improve mold release include addition to curable organosiloxane composition of any of a variety of internal release agents such as fatty acid metal salts such as zinc stearate and calcium stearate. This remedy does not provide a basic or fundamental solution to such problems as the poor vulcanization in the vicinity of the molding's parting line and surface tackiness.
Another method that has been used to cure both high consistency and liquid curable organosiloxane compositions is a hydrosilation reaction between an polyorganosiloxane containing at least two vinyl or other ethylenically unsaturated hydrocarbon radicals and an organohydrogensiloxane. This reaction is typically catalyzed by platinum metal and platinum compounds. One of the problems associated with this type of curable composition is the relatively short storage time of compositions containing all of the reactive ingredients, even when known platinum catalyst inhibitors are present in the curable composition.
The use of organic peroxides to accelerate curing of organosiloxane compositions that cure by a platinum-catalyzed hydrosilylation reaction is taught in U.S. Pat. No. 3,922,355, which issued to Itoh et al on Nov. 19, 1976. These compositions also contain hydrazine as a platinum catalyst inhibitor to extend the useful working time of the composition.
U.S. Pat. No. 4,329,275, which issued to Hatanaka et al. on May 11, 1982 teaches the use of small amounts of organic peroxides in combination with specified platinum/phosphorus complexes. The peroxide inhibits curing of the composition at room temperature while increasing the activity of the platinum catalyst at the elevated temperatures used to cure the composition.
The use of vinyl-specific organic peroxides to improve the compression set exhibited by elastomers prepared from liquid organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction is taught in U.S. Pat. No. 4,929,669, which issued to Jensen on May 29, 1990.
The present inventors discovered that when concentrations of peroxide larger than those disclosed in the aforementioned patent to Hatanaka et al. are present in the curable composition, a considerable amount of scorching occurs at even relatively low molding temperatures.
The use of encapsulated platinum-containing hydrosilation catalysts in place of conventional catalyst inhibitors to inhibit curing of organosiloxane compositions at temperatures of from 25 to about 40 degrees C. is taught in U.S. Pat. No. 4,481,341, which issued to Schlak et al. on Nov. 6, 1984. The shortcoming of the catalysts disclosed by Schlak is that the presence of even very small amounts of the platinum catalyst on the surface of the particles of encapsulated catalyst is sufficient to adversely affect the long-term storage stability of the composition.
U.S. Pat. No. 5,017,654 which issued on May 21, 1991 to Togashi and Saruyama teaches using actylenic compounds and certain alkenyl-functional organosiloxane compounds as platinum catalyst inhibitors to improve the storage stability of compositions containing encapsulated platinum catalysts.
An objective of this invention is to eliminate the disadvantages associated with using more than about 0.1 weight percent, based on curable organopolysiloxane, of organic peroxides in organosiloxane compositions curable by a platinum-catalyzed hydrosilation reaction when these compositions are used in applications requiring molding.